Bearing unit and rotation and drive device

ABSTRACT

The present invention relates to bearing unit for supporting a shaft  2  so as to freely rotate and includes a radial bearing  4  for supporting the shaft  2  so as to freely rotate and a housing member  6  made of a resin for holding the radial bearing  4.  The housing member  6  is formed with a material having a coefficient of thermal contraction larger than that of a material used for the radial bearing  4.  Assuming that the radial thickness of the radial bearing  4  is m and the radial thickness of a housing main body part of the housing member  6  with which the outer periphery of the radial bearing is covered is n, a relation of m&gt;n is satisfied. Thus, an influence due to a thermal contraction upon molding is prevented from being applied to the radial bearing.

TECHNICAL FIELD

The present invention relates to a bearing unit and a rotary drivingapparatus using this bearing unit, and more particularly to a bearingunit and a rotary driving apparatus using the bearing unit in which amechanical accuracy is maintained and a reliability is improved.

This application claims a priority based on Japanese Patent ApplicationNo. 2003-056696 filed in Mar. 4, 2003 in Japan, which is applied to thisapplication by referring thereto.

BACKGROUND ART

As a conventional bearing unit that accurately supports a rotary shaftand is excellent in its durability, there is, for instance, a bearingunit of a cooling fan used for cooling a heat generating device such asa CPU (a central processing unit) or a bearing unit of a rotary drumdriving motor used for a recording and reproducing apparatus using atape recording medium. As such a bearing unit, a bearing unit using afluid dynamic bearing as disclosed in Japanese Patent ApplicationLaid-Open No. 2000-205243 has been known. Further, the applicant of thisapplication proposes bearing units in the specifications and thedrawings of Japanese patent Application Laid-Open No. 2003-130043 orJapanese Patent Application Laid-Open No. 2003-232341.

A conventionally employed bearing unit has such problems as describedbelow from the viewpoint of reliability or mechanical accuracy.

For instance, in the bearing unit using a metallic housing member,component members are hardly completely combined or fastened to eachother and the leakage of lubricating oil is hardly assuredly prevented.Further, it is a complicated and expensive work to apply a polymerpacking material such as an adhesive to the entire periphery of afastening part without unevenness. Further, an inspection method forrecognizing whether or not the fastening part is completely sealedwithout a space is hardly obtained. As a result, a sufficientreliability cannot be obtained or an expensive facility is required.

Further, in the bearing unit using a housing member made of a resin, forinstance, when the housing member is formed by a material having acoefficient of thermal contraction higher than that of a material usedfor a radial bearing, a stress to the direction of an inside diametergenerated upon thermal contraction of the housing member is undesirablyadversely effected on the radial bearing. That is, a clearance requiredbetween a shaft and the radial bearing can not be sufficiently ensuredso that a mechanical accuracy may be possibly hardly maintained.

DISCLOSURE OF THE INVENTION

It is an object of the present invention to provide a new bearing unitand a rotary driving apparatus using the bearing unit that can solve theabove-described problems of a prior art.

It is another object of the present invention to provide a bearing unitcapable of assuring a mechanical accuracy between a shaft and a radialbearing for supporting the shaft and excellent in its durability and arotary driving apparatus using the bearing unit.

In a bearing unit according to the present invention proposed forachieving the above-described objects, when a housing member made of aresin for holding the radial bearing is formed with a material having acoefficient of thermal contraction larger than that of a material usedfor the radial bearing, assuming that the radial thickness of the radialbearing is m and the radial thickness of a part of the housing memberwith which the outer periphery of the radial bearing is covered is n, arelation of m>n is satisfied.

Further, the present invention concerns a rotary driving apparatus usingthe above-described bearing unit.

In the bearing unit according to the present invention, the radialbearing is held from its outer periphery by using the housing membermade of the resin. Further, the relation between the radial thickness mof the radial bearing and the radial thickness n of the part of thehousing member with which the outer periphery of the radial bearing iscovered satisfies m>n. Thus, a stress (compressive force) to thedirection of an inside diameter during the thermal contraction of thehousing member can be reduced to prevent the radial bearing from beingcompressed.

Still another objects of the present invention and specific advantagesobtained by the present invention will be more apparent from thefollowing description of embodiments made by referring to the drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a bearing unit according to thepresent invention.

FIG. 2 is a sectional view showing that the relation between thethickness m of a radial bearing and the thickness n of a housing membersatisfies m<n.

FIG. 3 is a sectional view showing another embodiment of a bearing unitaccording to the present invention.

FIG. 4 is a sectional view showing a still another embodiment of abearing unit according to the present invention.

FIG. 5 is a sectional view showing a still another embodiment of abearing unit according to the present invention.

FIG. 6 is a sectional view showing a rotary driving apparatus using thebearing unit according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

Now, a bearing unit according to the present invention and a rotarydriving apparatus using the bearing unit will be described below byreferring to the drawings.

Firstly, a first embodiment of the bearing unit according to the presentinvention will be described by referring to the drawings. As shown inFIG. 1, the bearing unit 1 has a shaft 2 formed by using a metallicmaterial such as stainless steel or a resin material and a bearingmechanism 3 for supporting the shaft 2. Here, as the shaft 2, a rotaryshaft supported by the bearing mechanism 3 so as to freely rotate isused. Further, the bearing mechanism 3 includes a radial bearing 4 forreceiving a radial load exerted on the shaft 2 and a thrust bearing 5for receiving a thrust load. The bearing mechanism 3 is housed in ahousing member 6 serving as a support member of the shaft 2, or formedas a part of the housing member 6.

Then, as the radial bearing 4 for supporting the shaft 2 so as to freelyrotate with respect to a radial direction, for instance, an oilimpregnated sintered bearing or a fluid dynamic bearing is used. Thefluid dynamic bearing used here is specifically explained. The fluiddynamic bearing is formed by molding copper based or copper-iron basedsintered metal in a cylindrical form and has two sets of grooves 4 a and4 b for generating dynamic pressure formed on its inner peripheralsurface. These dynamic pressure generating grooves 4 a and 4 b areformed by successively extending V shaped grooves in the direction of acircumference. Further, the fluid dynamic bearing is impregnated withlubricating oil by employing the porous structure of the sintered metalforming the bearing.

In this embodiment, the dynamic pressure generating grooves 4 a and 4 bforming the fluid dynamic bearing are formed on the inner peripheralsurface of the radial bearing 4, however, the grooves may be formed onthe outer peripheral surface of the shaft 2 supported by the radialbearing 4.

In this embodiment, the two sets of the dynamic pressure generatinggrooves 4 a and 4 b are provided in parallel in the axial direction onthe inner peripheral surface of the radial bearing 4.

Further, as the thrust bearing 5 for supporting the shaft 2 in thethrust direction, a pivot bearing or a fluid dynamic bearing is used. Inthe embodiment shown in FIG. 1, for the thrust bearing 5, the pivotbearing is used that the end part 2 a of the shaft 2 formed in aprotruding curved surface such as a spherical surface is supported by asupport surface 7 of the housing member 6. In this embodiment, thehousing member 6 forms a part of the thrust bearing 5. That is, asupport member for supporting the end part 2 a of the shaft 2 may beformed separately from the housing member 6. However, the support memberis formed integrally with the housing member 6 so that the number ofparts can be reduced and a manufacturing cost can be reduced.

The housing member 6 having the radial bearing 4 housed therein and thethrust bearing 5 also functions to hold lubricating oil with which a gapformed between the shaft 2 and the radial bearing 4 and the trustbearing 5 for supporting the shaft 2 is filled. Accordingly, the housingmember 6 is formed with a material capable of preventing the leakage ofthe lubricating oil. Specifically, the housing member 6 is formed bymolding a polymer material such as nylon (straight chain aliphaticpolyamide), liquid crystal polymer (LCP), polyimide, or the like.

The housing member 6 is formed in the cylindrical shape with a bottom byusing a polymer material having a coefficient of thermal contractionlarger than that of the sintered metal forming the radial bearing 4.Namely, the housing member 6 includes a lubricating oil seal part 8, ahousing main body part 9 in the outer peripheral side of the radialbearing 4, and a bottom part 10 by which the thrust bearing 5 is formed.A gap G is formed between the inner peripheral surface 8 a of thelubricating oil seal part 8 and the shaft 2.

In this invention, when the radial thickness of the radial bearing 4 isset to m and the radial thickness of the housing main body part 9forming the housing member 6 is set to n, a relation of m>n isestablished between them. That is, the thickness n of the housing mainbody part 9 with which the outer periphery of the radial bearing 4 iscovered is smaller than the thickness m of the radial bearing 4 in theradial direction from the shaft 2 as a center.

In the bearing unit 1 according to the present invention, an outsertmolding is carried out by arranging the radial bearing 4 in a metal moldfor forming the housing member 6 made of the polymer material. Thus, theradial bearing 4 can be easily and highly accurately arranged in thehousing member 6. Further, a part of the housing member 6 is used toform the thrust bearing 5 and the lubricating oil seal part 8 is formedintegrally with the housing member 6. Thus, the number of parts or thenumber of manufacturing steps can be reduced and the manufacturing costcan be reduced.

Further, the housing member 6 for housing and supporting the bearingmechanism 3 has an integral seamless structure. Thus, the leakage of thelubricating oil can be prevented and the bearing unit excellent in itsreliability can be formed.

Here, the above-described relation of m>n will be described below. Sincethe housing member 6 is ordinarily formed with the polymer materialhigher in the coefficient of thermal contraction than metal, a stressexerted on the radial bearing 4 upon thermal contraction in a moldingprocess causes a problem.

For instance, when the housing member 6 is formed by outsert molding inthe outer periphery of the radial bearing 4 formed by using the sinteredmetal made of copper or iron, assuming that a relation of m<n isestablished as shown in FIG. 2, high molding temperature is cooled toordinary temperature. At this time, the housing main body part 9 of thehousing member 6 compresses the radial bearing 4 located in the innerperipheral side thereof in the radial direction, that is, toward adirection shown by an arrow mark F coming near to the shaft 2 in FIG. 2.Thus, the inside diameter of the radial bearing 4 is undesirablycontracted.

Since a radial clearance between the shaft 2 and the radial bearing 4for supporting the shaft 2 ordinarily needs to be held to about 1 μm to10 μm, and desirably to about several μm, the large contraction of theinside diameter of the radial bearing 4 causes an unallowable problem tothe bearing unit.

Thus, in the present invention, the relation between the radialthickness m of the radial bearing 4 and the radial thickness n of thehousing main body part 9 of the housing member 6 satisfies m>n.Consequently, a quantity of thermal contraction of the housing member 6is reduced and the rigidity of the radial bearing 4 is improved in arelative relation to the housing member 6. Accordingly, even when thehousing member 6 is outsert-molded in the periphery of the radialbearing 4 by using the polymer material or the like, the inside diameterof the radial bearing 4 is not contracted by the thermal contraction ofthe housing member 6. Therefore, a highly precise mechanical accuracycan be maintained, and a good lubrication to the shaft 2 and the stablerotation of the shaft 2 can be realized.

The fact that the radial thickness m of the radial bearing 4 and theradial thickness n of the housing main body part 9 of the housing member6 establish the relation of m>n can be obtained under a condition that aquantity of radial contraction of the radial bearing 4 is not lower thana quantity of radial contraction of the housing member 6 in the radialdirection of the shaft 2 as a center on the assumption that a materialforming the housing member 6 has a coefficient of linear expansionlarger than that of a material forming the radial bearing 4. Theabove-described relation is not directly related to kinds of materialsforming the radial bearing 4 or the housing member 6.

In this embodiment, to prevent the leakage of the lubricating oil in apart in which the shaft 2 protrudes to an outer part from the end of thehousing member 6, a part that forms the gap G between the innerperipheral surface 8 a of the seal part 8 and the shaft 2 is formed as atapered part 2c in which the diameter is reduced along the shaft 2toward the end and the diameter is enlarged as the shaft comes near tothe radial bearing 4 in the inner direction of the housing member 6.That is, the gap G is formed between the tapered part 2 c formed so thatthe diameter becomes gradually large toward the inner part and the innerperipheral surface 8 a of the seal part 8 opposed thereto. Accordingly,a quantity of gap is gradually decreased toward the inner part of thehousing member 6. Then, pull-in pressure generated due to a capillaryaction is inversely proportional to the quantity of gap. Thus, as thequantity of gap is more decreased, the generated pull-in pressure is themore increased. Thus, the lubricating oil existing in the gap is pulledin the inner part of the housing member 6 having a small quantity ofgap. Accordingly, the lubricating oil can be prevented from movingoutside and leaking. Further, the bias of the lubricating oil due toeccentricity is more effectively reduced than a case that the diameterof a hole is constant. Further, the lubricating oil can be effectivelyprevented from being scattered outside by the action of a centrifugalforce upon rotation of the shaft 2.

Now, another embodiment of a bearing unit according to the presentinvention will be described below by referring to FIGS. 3 to 5.

A bearing unit 11 shown FIGS. 3 and 4 uses a pivot bearing as a thrustbearing. A bearing unit 11 shown in FIG. 5 uses a fluid dynamic bearingas a thrust bearing.

In the bearing unit 11 shown in FIG. 3, an end of a shaft 12 is workedto a spherical part and the spherical part is supported by the thrustbearing formed with a polymer material.

The bearing unit 11 shown in FIG. 3 includes a shaft 12 formed by usinga metallic material such as stainless steel and a bearing mechanism 13for supporting the shaft 12. Here, as the shaft 12, a rotary shaftsupported by the bearing mechanism 13 so as to freely rotate is used.Further, the bearing mechanism 13 includes a radial bearing 14 forreceiving a radial load exerted on the shaft 12 and a thrust bearing 15for receiving a thrust load. The bearing mechanism 13 is housed in ahousing member 20 serving as a support member of the shaft 12.

Then, as the radial bearing 14 for supporting the shaft 12 so as tofreely rotate with respect to a radial direction, for instance, asintered oilless bearing or a fluid dynamic bearing is used. The fluiddynamic bearing used here is specifically explained. The fluid dynamicbearing is formed by molding copper based or copper-iron based sinteredmetal in a cylindrical form and has two sets of grooves 14 a and 14 bfor generating dynamic pressure formed on its inner peripheral surface.These dynamic pressure generating grooves 14 a and 14 b are formed bysuccessively extending V shaped grooves in the direction of acircumference. Further, the fluid dynamic bearing is impregnated withlubricating oil by employing the porous structure of the sintered metalforming the bearing.

In this embodiment, the two sets of the dynamic pressure generatinggrooves 14 a and 14 b forming the fluid dynamic bearing are formed onthe inner peripheral surface of the radial bearing 14, however, thegrooves may be formed on the outer peripheral surface of the shaft 12supported by the radial bearing 14.

In this embodiment, the two sets of the dynamic pressure generatinggrooves 14 a and 14 b are provided in parallel in the axial direction onthe inner peripheral surface of the radial bearing 14.

In the end side of the shaft 12 supported by the bearing mechanism 13,an annular engaging groove 12 a is formed. A slip-off preventing member16 is attached to the engaging groove 12 a. The slip-off preventingmember 16 is made of, for instance, a polymer material such as nylon ora metallic material. The slip-off preventing member 16 functions as astopper for preventing the shaft 12 from moving toward a central axialdirection and slipping off when external force is axially exerted due toa vibration or the change of atmospheric pressure is generated.

In the periphery of the slip-off preventing member 16, a member formedby using polymer materials such as nylon, polyimide, liquid crystalpolymer or metal or the like, that is, a space forming member 17 isprovided. The space forming member 17 is arranged to form a prescribedspace in the periphery of the slip-off preventing member 16 byconsidering that the slip-off preventing member 16 is fixed to the shaft12 and rotates together with the shaft 12.

In this embodiment, the space forming member 17 made of a syntheticresin is formed in a bottomed tubular form having a recessed part 17 a.The spherically formed end of the shaft 12 comes into point-contact witha bottom surface of the recessed part 17 a formed as a flat surface. Asdescribed above, a protruding curved surface is formed on the end 12 bof the shaft 12 and comes into contact with the space forming member 17.Thus, a part of the space forming member 17 can form the thrust bearing15. Accordingly, the thrust bearing does not need to be independentlyprovided. Thus, a structure as the bearing unit 11 can be simplified,the number of parts can be reduced and a manufacturing cost can bereduced.

In the bearing unit 11 according to the present invention, a protrudingpart may be formed in the space forming member 17 side to support theend of the shaft 12 formed as a flat surface.

In an original space forming member 17, a step part 17 b is formed. Thisstep part 17 b forms a receiving recessed part to which the radialbearing 14 is partly fitted.

A seal member 18 for sealing lubricating oil is disposed with a verysmall gap G formed between an inner peripheral surface 18 a and thetapered part 12 c of the shaft 12. The seal member is formed in acylindrical shape by using a polymer material such as nylon orpolytetrafluoroethylene or metal. In this seal member 18, a step part 18b is formed. This step part 18 b forms a receiving recessed part towhich the radial bearing 14 is partly fitted. A recess 18 c formed inthe seal member 18 is formed so as to correspond to a protruding partformed in the end part of the radial bearing 14. This protruding partserves as an index to discriminate a direction in the axial direction.The gap G is filled with lubricating oil 19.

The housing member 20 is formed by outsert-molding a synthetic resinsuch as a polymer material. In this embodiment, the housing member 20serves to completely fasten the radial bearing 14, the space formingmember 17 and the seal member 18 in a seamless manner without gaps.Thus, the leakage of the filled lubricating oil is prevented.

In this embodiment, between the radial thickness n of a housing mainbody part 20 a of the housing member 20 with which the outer peripheryof the radial bearing 14 is covered and the radial thickness m of theradial bearing 14, a relation of m>n is established in the same manneras that of the above-described bearing unit 1.

Now, a method for manufacturing the bearing unit 11 shown in FIGS. 3 and4 will be briefly described.

To manufacture the bearing unit 11, the shaft 12 to which the slip-offpreventing member 16 is attached is firstly inserted into the radialbearing 14 in a shaft inserting process.

Then, in an attaching process of the space forming member 17 and theseal member 18, the step part 17 b of the space forming member 17 or thestep part 18 b of the seal member 18 is fitted to the outer peripheraledge of each of end parts in the axial direction of the radial bearing14. Thus, the radial bearing 14 is partly fitted to each of the recessedparts of the space forming member 17 and the seal member 18. When thisprocess is finished, the shaft 12 is already supported by the bearingmechanism 13 so as to freely rotate.

Then, in a forming process of the housing member 20, the housing member20 is formed by outsert molding operation using the polymer material sothat the relation between the radial thickness m of the radial bearing14 and the radial thickness n of the housing main body part 20 a formingthe housing member 20 satisfies a condition of m>n. After that, the unitis filled with the lubricating oil by vacuum pressure impregnation in alubricating oil filling and oil quantity adjusting process to adjust anoil quantity. The oil quantity is adjusted by removing excessive oildischarged outside by a thermal expansion, for instance, under thecondition of prescribed temperature.

In the bearing unit 11 formed in such a way, a packing applied to thefastening part of the members as in the conventional bearing unit doesnot need to be managed so that a schedule control is simplified.

The above-described space forming member 17 is not limited to the membermade of the synthetic resin and may be made of metal.

The bearing unit using the pivot bearing as the thrust bearing may beformed as shown in FIG. 4.

In a following description, parts common to those of the bearing unit 11shown in FIG. 3 are designated by the same reference numerals and adetailed description thereof is omitted.

In a bearing unit 11A shown in FIG. 4, a space forming member 17A isformed by using metallic materials such as stainless steel, brass,pressed materials, sintered materials, etc.

Further, a thrust bearing 15 has a thrust bearing member 21 forreceiving the end 12 b of a shaft 12 worked to a spherical surfaceshape. The thrust bearing member 21 is attached to the recessed part 17a of the space forming member 17A. The thrust bearing member 21 isformed separately from the space forming member 17A by using a resinmaterial such as nylon, polyimide, polyamide, liquid crystal polymer,etc. or a low friction material such as rubidium.

In the bearing unit 11A shown in FIG. 4, since the space forming member17A is made of metal, the thrust bearing member 21 using the syntheticresin material or the low friction material is provided to realize along life. Then, the rigidity of the space forming member 17A isimproved and the space forming member has a structure capable ofwithstanding high temperature. Thus, conditions such as the fillingtemperature of a resin or pressure, etc. in an outsert molding processof a housing member 20 that is performed after the space forming member17A is attached are mitigated. Namely, in this embodiment, there is afear that a cost is increased because of the thrust bearing member 21.However, the resin material to be used is not selected and moldingconditions are mitigated, so that a whole manufacturing cost can bereduced.

FIG. 5 shows still another embodiment of a bearing unit according to thepresent invention. The difference between a bearing unit 11B of thisembodiment and the bearing unit 11 shown in FIG. 3 resides in thedifference in the structure of the shaft 12 to be supported.

The shaft 12 used in the bearing unit 11B shown in FIG. 5 has an end ofthe shaft which is T-shaped in side view. A slip-off preventing memberof the shaft 12 is used to form a fluid dynamic bearing. Accordingly,parts common to those of the bearing unit 11 shown in FIG. 3 aredesignated by the same reference numerals and a detailed descriptionthereof is omitted.

In the bearing unit 11B shown in FIG. 5, the slip-off preventing member22 provided in the end of the shaft 12 is formed in a disc having aprescribed thickness and made of metal such as brass or stainless steel,or polymer materials such as nylon, LCP, etc. On both end faces in theaxial direction of the slip-off preventing member 22, that is, on a face23 opposed to a radial bearing 14 and a face 24 opposed to a spaceforming member 17, dynamic pressure generating grooves 23 a and 24 a arerespectively formed.

In the space forming member 17, a recessed part 17 a for receiving theslip-off preventing member 22 is formed. Thus, a space is formed in theperiphery of the slip-off preventing member 22. A gap formed between theslip-off preventing member 22 and the space forming member 17 or a gapformed between the slip-off preventing member 22 and the radial bearing14 is filled with lubricating oil.

As described above, the bearing unit 11B shown in FIG. 5 has a structureof a fluid dynamic bearing type using the slip-off preventing member 22and the space forming member 17 as a thrust bearing 15. Since the shaft12 is supported to relatively freely rotate by the fluid dynamicbearing, a vibration is reduced. Accordingly, the bearing unit ispreferably suitably used for a driving motor for a recording/reproducingdevice such as an optical disc drive or a hard disc drive.

Also in this embodiment, the radial thickness n of a housing main bodypart 20 a of a housing member 20 with which the outer periphery of theradial bearing 14 is covered and the radial thickness m of the radialbearing 14 satisfy the relation of m>n.

Further, in this embodiment, the dynamic pressure generating grooves 23a and 24 a are formed on the slip-off preventing member 22. However, thepresent invention is not limited thereto, and the dynamic pressuregenerating grooves may be formed on an end face of the radial bearing 14opposed to the slip-off preventing member 22 or a face of the spaceforming member 17 opposed to the slip-off preventing member 22.

Now, a rotary driving apparatus using the bearing unit according to thepresent invention will be described below by referring to FIG. 6.

A rotary driving apparatus 25 shown in FIG. 6 specifically forms a fanmotor of a personal computer.

The rotary driving apparatus 25 shown in FIG. 6 includes a rotor part 26and a stator part 27 using the bearing unit 11 shown in FIG. 3.

The rotor part 26 forming a rotor includes a rotor yoke 28, a magnet 29and a plurality of fan vanes 30. An end part of a rotating shaft 12 isfitted under pressure and fixed to a boss part 31 formed at a positionas a center of rotation. Then, to the inner peripheral surface of therotor yoke 28, the annular magnet 29 magnetized along the direction of acircumference is bonded and fixed. On the outer peripheral surface of acylindrical part 26 a forming the rotor part 26, the plurality of fanvanes 30 are provided at intervals of prescribed angles along thedirection of the circumference. Here, as the magnet 29, a plastic magnetis used.

The bearing unit 11 is disposed in the stator part 27 as shaftsupporting means for supporting the shaft 12 rotating together with therotor part 26 so as to freely rotate. That is, the bearing unit 11 isfitted to a recessed part 33 of a cylindrical support part 32 a formedin a stator yoke 32 forming the stator part 27 and further fixed theretoby using an adhesive. A coil part 36 including a core 34 and a coil 35is provided at a position of an outer peripheral part of the supportpart 32 a opposed to the inner peripheral surface of the magnet 29 andforms a driving part 37 of the rotor together with the magnet 29 and therotor yoke 28.

A hole 38 a is formed on a case 38 of the rotary driving apparatus 25.When the rotor part 26 is rotated by supplying electric current to thecoil part 36, air enters from the hole 38 a as shown by an arrow mark Ain FIG. 6, and then, is discharged outside the case 38 from an airsupply port (not shown) formed in the case 38.

As described above, the bearing unit 11 according to the presentinvention is mounted on the rotary driving apparatus 25, so that therotary driving apparatus 25 having no leakage of lubricating oil andlong life and excellent in its reliability can be realized. Further, thefluid dynamic bearing is used as the radial bearing 14, so that therotary driving apparatus 25 having no leakage of lubricating oil andhigh reliability and capable of realizing a high speed rotation can beformed. Accordingly, the rotary driving apparatus may be advantageouslyapplied to a cooling fan of a heat generating device that requires ahigh cooling performance.

Further, when the rotary driving apparatus 25 according to the presentinvention is applied to a cooling system of a heat generator such as aCPU used for a computer, the rotary driving apparatus can be applied toa cooling mechanism which transmits heat generated from the heatgenerator to a heat sink, and carries out air cooling of this heat sinkby a fan.

The rotary driving apparatus 25 according to the present invention maybe installed irrespective of upper and lower directions along the shaft12. Accordingly, the rotary driving apparatus can be installed in anelectronic device such as a personal computer by inverting upper andlower parts from a state shown in FIG. 6.

The rotary driving apparatus 25 according to the present invention isnot limited to a cooling fan motor and may be widely applied to arotating device of a disc type recording medium or a driving motor of arotary type head drum device or the like.

The rotary driving apparatus 25 according to the present invention canuse either the bearing unit 11, 11A or 11B.

As described above, in the bearing unit according to the presentinvention, the housing member is formed by using the polymer materialand has the coefficient of thermal contraction relatively larger thanthat of the radial bearing made of the sintered metal or the like andsupported by the housing member. A condition of n<m that the radialthickness n of the housing member is smaller than the radial thickness mof the radial bearing is satisfied. Thus, when the housing member isoutsert-molded, the stress to the direction of the inside diameter dueto the thermal contraction of the housing member is reduced. Therefore,the bearing unit in which the accuracy of inside diameter of the radialbearing can be sufficiently maintained, a necessary clearance is assuredbetween the shaft and the radial bearing and loss torque is decreasedcan be realized.

Further, in the bearing unit according to the present invention, a goodlubrication and long life can be obtained and reliability can beimproved without aged deterioration.

Further, since the thickness of the housing member formed by molding thesynthetic resin is small, the dimensional accuracy of its outsidediameter is easily maintained.

Still further, when the bearing unit according to the present inventionis attached to the device such as the driving motor, the bearing unitcan be accurately fixed to the device by simply fitting it to a part ofthe device and a mechanical accuracy related to a rotation can beimproved. When the bearing unit is applied to the above-described rotarydriving apparatus, a relative positional relation between the magnet andthe coil part can be satisfactorily maintained and a stable magneticcircuit can be obtained.

Particularly, in the bearing unit according to the present invention,the fluid dynamic bearing is used for the radial bearing. Thus, assumingthat a quantity of gap between the shaft and the bearing is c and thedepth of the dynamic pressure generating groove is h, (c+h)/c is veryimportant. The value of a load capacity depends on the value of thisratio. That is, when the value of the ratio is lower than a certaintolerance or when the value of the ratio exceeds the tolerance, thedynamic pressure is reduced. Thus, whether or not the performance of thefluid dynamic bearing is exhibited as designed depends on themaintenance of the accuracy of the quantity of gap c. In the bearingunit according to the present invention, since the effect of the stressto the bearing upon thermal contraction can be eliminated to assure aprescribed quantity of gap, the shaft can be highly accurately supportedand the stable rotation of the shaft can be assured.

Still further, since the radial bearing is relatively thicker than thehousing member, the sufficient rigidity of the housing member isobtained. Accordingly, the resin material forming the housing member iseasily selected and the conditions upon molding are easily set.

The present invention is not limited to the above-described embodimentsexplained with reference to the drawings. It is apparent to a personwith ordinary skill in the art that various changes, substitutions orequivalence thereto may be made without departing the attached claimsand the gist thereof.

Industrial Applicability

As described above, in the bearing unit according to the presentinvention, the mechanical accuracy of the inside diameter of the radialbearing for supporting the shaft can be easily maintained, the shaft canbe highly accurately supported and the stable rotation of the shaft canbe assured. The stable rotation of the rotary driving apparatus usingthe bearing unit can be assured.

1. A bearing unit having a shaft, a radial bearing for supporting theshaft so as to freely rotate and a housing member made of a resin forholding the radial bearing, wherein the housing member is formed with amaterial having a coefficient of thermal contraction larger than that ofa material used for the radial bearing, and assuming that the radialthickness of the radial bearing means is m and the radial thickness of apart of the housing member with which the outer periphery of the radialbearing means is covered is n, a relation of m>n is satisfied.
 2. Thebearing unit according to claim 1, wherein a thrust bearing forreceiving a thrust load exerted on the shaft is provided and the radialbearing and the thrust bearing are held by the housing member formed byusing a resin material.
 3. The bearing unit according to claim 1,wherein a fluid dynamic bearing is used as the radial bearing means. 4.The bearing unit according to claim 1, wherein a polymer material isused for the housing member.
 5. A rotary driving apparatus including arotor and a shaft rotating together with the rotor, a radial bearingmeans for supporting the shaft so as to freely rotate, a housing membermade of a resin for holding the radial bearing means and a driving meansfor rotating the rotor, wherein the housing member is formed with amaterial having a coefficient of thermal contraction larger than that ofa material used for the radial bearing means, and assuming that theradial thickness of the radial bearing means is m and the radialthickness of a part of the housing member with which the outer peripheryof the radial bearing means is covered is n, a relation of m>n issatisfied.
 6. The rotary driving apparatus according to claim 5, whereina thrust bearing means for receiving a thrust load exerted on the shaftis provided and the radial bearing and the thrust bearing are held bythe housing member formed by using a resin material.
 7. The rotarydriving apparatus according to claim 5, wherein a fluid dynamic bearingis used as the radial bearing means.
 8. The rotary driving apparatusaccording to claim 5, wherein a polymer material is used for the housingmember.